1. Field of the Invention
This invention relates to display devices, for example display devices using illumination light sources, with the display device modulating the light from the illumination light source.
2. Description of the Related Art
A liquid crystal display is the most common example of this type of modulating display device, and typically comprises an active plate and a passive plate between which liquid crystal material is sandwiched. The active plate comprises an array of transistor switching devices, typically with one transistor associated with each pixel of the display. Each pixel is also associated with a pixel electrode on the active plate to which a signal is applied for controlling the brightness of the individual pixel.
The level of ambient light has a strong influence on the performance of a display device which is used to modulate a light source.
It has been recognised that the performance of displays can be improved by using information from light sensors to modify the operation of the display. For example, the intensity of the backlight of the display may be adjusted in response to information from light sensors which are able to sense the characteristics of the ambient illumination as a means of reducing the power consumption of the display when the ambient light levels are low, and to provide a good quality output when the ambient light levels are high.
The required light sensors can be formed as part of the active plate using thin film technology, and this is a convenient way of adding the light sensor capability without requiring additional process steps or separate components. The light sensitive devices may for example be thin film transistors, thin film diodes, lateral diodes or light sensitive resistors.
However, in the case where the display makes use of a light source for illumination (this may be a backlight or a frontlight) it can be difficult to optically isolate the light sensors from this light source.
This problem is illustrated in FIG. 1 which shows a display system having a display 10, a backlight 12, a light sensor 14 and control circuitry 16 for operating the display and the backlight. A signal is fed from the light sensor 14 to the controller 16 so that the controller can modify the operation of the display and the backlight in response to changes in the detected illumination.
There will be contributions to the output signal from the sensor 14 which result from the ambient light 18 at the front of the display and from the light 20 generated by the backlight 12. In order to correctly adjust the operation of the display and backlight, it is necessary to differentiate between the light from these two sources.
WO 20007/069107 discloses a system in which light sensors are used to enable both ambient illumination levels and backlight output levels to be measured.
FIG. 2 shows in a simplified form the way in which the light sensor can be integrated within the display. In this example, the display is formed from two glass substrates 24, 26 with a liquid crystal layer 28 between them. In this example, the light sensor is arranged as an array of light sensor elements 30 which are fabricated on the lower substrate 26 which is closest to the backlight 42 (or backlight light guide) of the display. The sensor might be a thin film diode, thin film transistor or other photosensitive device. Ambient light from the front of the display is able to pass through the upper substrate 24 and the liquid crystal layer 28 to reach the light sensor 30.
The sensor can also receive ambient light which has passed through the display and has been modulated by the display pixels as indicated by the example light path 31. The sensor may also receive light from the backlight of the display as indicated by light paths 32 and 34, and which has passed through the lower substrate 26.
When measuring the ambient illumination, the contributions to the output signal from the modulated ambient light and from the backlight are undesirable and should be minimised and ideally eliminated.
It is possible to block the direct path of light from the backlight to the light sensor, for example by providing an opaque layer at the base of the thin film layers defining the light sensor. However, light from the backlight will be reflected or guided within the substrates of the display and will therefore still reach the sensors via an indirect path. This indirect light path is shown by arrows 32, whereas the direct path is shown as 34.
For completeness, FIG. 2 shows a light masking layer 36. The use of a black mask layer is well known to shield the areas of the active plate through which unmodulated light can pass, and to shield the transistors as their operating characteristics are light-dependent. The top and bottom polarizers 38, 40 are also shown. The black mask layer has an opening to allow ambient light to reach the sensor 30.
The light sensors can be integrated within the display pixels, or a smaller number of light sensor devices may be provided at the edge of the pixel array.
Another problem faced when integrating ambient light sensors on display substrates is that the ambient light level can vary over a very wide range, from more than 100,000 lux in direct sunlight down to just a few lux at night or in a darkened room. When measuring low light levels, the leakage current (dark current) of the photodiode or phototransistor is a significant source of errors. At low and medium light levels, in the case of an LCD, light from the backlight or front light can significantly alter the output signal of the sensor which may prevent the ambient light level from being measured.
This can be avoided by turning off the backlight or front light during the measurement, but at high ambient light levels the light source should be operated continuously to maximise the display brightness.
In order to measure the ambient light level under these different conditions it is necessary to change the way in which the measurement is performed. This can lead to a discontinuity in the output of the measurement when the measurement mode is changed.